This invention relates to nuclear reactors and has particular relationship to calandrias for use in nuclear reactors. Veronesi et al discloses a nuclear reactor in which the coolant flowing through the upper internals is conducted to the outlet nozzles through a calandria which is at the level of the nozzles. It is desirable that the calandria be a unit readily installed in, and removable from, the nuclear reactor. The calandria should include facilities for locating and positioning the control-rod guides at their upper ends radially with respect to the reactor. It is necessary that the calandria provide guidance for the drive rods for the control rods between the top of the rod guides and the head of the pressure vessel of the reactor and facilitate the insertion of the drive rods into the guides. The drive rods should be protected from the transverse flow of the coolant. The calandria should provide structural support for the drive line during a loss-of-coolant accident (LOCA) and insure that the control rods will be capable of being inserted in the core during LOCA events. It is an object of this invention to provide a calandria which, while suitable for other purposes, is uniquely suitable for use in a reactor, such as that disclosed in Veronesi et al, wherein the calandria is mounted above the upper internals, and meets the above-listed requirements.